This invention relates to a power assisted steering gear and is particularly concerned with such a gear having a toothed part which is coupled to an axially extending input shaft and which is rotatably mounted for driving an output member in response to rotation of the input shaft about its axis. In such a gear the toothed part is usually in the form of a pinion or worm rotation of which drives a toothed rack member or toothed sector member which comprises, or is coupled to, the output member. Conventionally a servo motor is associated with the output member to provide power assistance to displacement of that member and the flow of fluid under pressure to the servo motor is controlled by a valve which reacts between the toothed part and the input shaft and is responsive to rotation of the input shaft. Typically the valve and toothed part of a power assisted steering gear are operatively mounted in a cavity formed by a steering gear housing with the input shaft extending from a side of the housing for connection, usually, to a steering column. It has hitherto been proposed with such a typical arrangement for the toothed part and the valve to be locatable in, and removable from, the cavity by way of a side port in the housing and for the toothed part and valve to be operatively coupled together following the insertion of the latter of these components in the cavity. Generally the toothed part is inserted first into the cavity for engagement with a further toothed part where it is rotatably mounted and retained, such retention being by means of spring clips, thrust bearings or the like. Subsequent to the retention of the toothed part in the cavity, the valve is inserted to operatively engage with the toothed part and to be rotationally mounted and retained in the cavity (the retaining of the valve again being effected by the use of thrust bearings, retaining clips, caps or plates which may close the cavity). The assembly of such a prior proposal in piecemeal manner and likewise the dis-assembly of the gear for servicing or replacement of the components is often inconvenient and time consuming (particularly where the toothed part has to be removed from the cavity and it is necessary first to remove the valve part and thereafter to remove the retaining means for the toothed part which means is generally regarded as somewhat inaccessible and awkward to engage within the cavity). It is an object of the present invention to provide a power assisted steering gear which comprises a toothed part and a valve mounted in a cavity of a gear housing and which gear facilitates assembly and dis-assembly of the valve and toothed part components with the cavity of the housing and alleviates the disadvantages as previously described of the prior proposals.